1. Field of the Invention
This invention relates to a machine for producing a fibrous web, in particular a paper web, paperboard web or tissue web, having a former on which the fibrous web is formed on a structured belt and is dewatered between said structured belt and a forming belt, and having a drying apparatus for the further dewatering of the fibrous web, through which the fibrous web is guided together with the structured belt and in which hot air flows through the permeable structured belt and the fibrous web. Such a machine is described for example in WO 2005/075737 A1.
2. Description of the Related Art
Also known already from WO 00/75423 A1 is a so-called printed forming fabric.
Such a printed forming fabric can be used for example on conventional tissue machines. However, there is no advantage in using a printed forming fabric for an application on such conventional tissue machines because the sheet is pressed 100% and the volume is too small to produce a micro-embossed and macro-embossed sheet on the machine. A suitable processing plant is required for embossing the sheet.
A printed forming fabric can also be used on a TAD machine (TAD=Through Air Drying) where the volume and the sheet absorption capacity are 50 to 100% greater than on conventional machines.
As can be seen from FIG. 1, on such a TAD machine the sheet is formed between two mesh belts 10, 12. FIG. 1 shows in a schematic partial representation the forming region of such a TAD machine. FIG. 2 shows in a schematic representation the TAD machine together with the TAD cylinder 14. As is evident in particular from FIG. 2, on such a TAD machine the fibrous web is dewatered by way of a vacuum directly following the forming region, whereby said dewatering is performed up to a dry content of between 22 and 26%. Not until such a high consistency is reached is the fibrous web then transferred from the one forming mesh or belt 10, which is a smooth belt, to an embossing or structured belt 16, where it is wet-embossed by way of a vacuum box or wet embossing box which sucks the fibers into the depressions of the structured belt 16.
Because a vacuum is applied to an already formed fibrous web with a consistency of over 20%, the fibers are stretched into the depressions, as the result of which the sheet thickness is reduced and only a small part of the fibers remains protected inside the structure of the belt, whereby the fibers in question are those which are not pressed in order to achieve a certain quality (cf. FIGS. 1 and 2). Hence a negative draw between the forming zone and the TAD zone is required on such a TAD machine. Usually, TAD machines are operated in the TAD section at a 20% lower speed in order to brush the fibers into the depressions of the belt. As the result, the entire macro-embossing (markings) from the printed forming fabric are destroyed again by the difference in speed between the forming zone and the TAD zone. On such TAD machines the macro-embossing and micro-embossing take place with the structured belt in the TAD zone instead of in the forming zone.
With such a micro-embossing and macro-embossing on the machine it would be possible to avoid performing such an embossing on the processing plant and hence compressing the sheet and sacrificing quality.
On a TAD machine, quality is thus produced in the TAD zone. The negative draw (˜0.8 V) on such a machine serves to produce quality but destroys the embossing effect. Also evident in FIG. 2 are the regions 18 in which the fibrous web, after being transferred from the structured belt 16 to the Yankee cylinder 14, has no contact with the Yankee cylinder.
What is needed in the art is a machine of the type initially referred to which in terms of the quality, volume and water absorption capacity of the produced fibrous web is comparable to a TAD machine.